It is important to increase the yield of chemical pulp for effective utilization of wood resources. A polysulfide cooking process is one of techniques to increase the yield of kraft pulp as the most common type of chemical pulp.
The cooking liquor for the polysulfide cooking process is produced by oxidizing an alkaline aqueous solution containing sodium sulfide, i.e. so-called white liquor, by molecular oxygen such as air in the presence of a catalyst such as activated carbon (e.g. reaction formula 1) (JP-A-61-259754 and JP-A-53-92981). By this method, a polysulfide cooking liquor having a polysulfide sulfur concentration of about 5 g/l can be obtained at a selectivity of about 60% and a conversion of 60% based on the sulfide ions. However, by this method, thiosulfate ions not useful for cooking, are likely to form by side reactions (e.g. reaction formulae 2 and 3), whereby it used to be difficult to produce a cooking liquor containing polysulfide sulfur at a high concentration with a high selectivity. EQU 4Na.sub.2 S+O.sub.2 +2H.sub.2 O.fwdarw.2Na.sub.2 S.sub.2 +4NaOH (1) EQU 2Na.sub.2 S+20.sub.2 +H.sub.2 O.fwdarw.Na.sub.2 S.sub.2 O.sub.3 +2NaOH (2) EQU 2Na.sub.2 S.sub.2 +30.sub.2.fwdarw.2Na.sub.2 S.sub.2 O.sub.3 (3)
Here, polysulfide sulfur which may also be referred to as PS-S, is meant for sulfur of 0 valency in e.g. sodium polysulfide Na.sub.2 Sx, i.e. sulfur of (x-1) atoms. Further, in the present specification, sulfur corresponding to sulfur having oxidation number of -2 in the polysulfide ions (sulfur of one atom per Sx.sup.2-) and sulfide ions (S.sup.2-) will generically be referred to as Na.sub.2 S-state sulfur. In the present specification, the unit liter for the volume will be represented by l.
On the other hand, PCT International Publication WO95/00701 discloses a method for electrolytically producing a polysulfide cooking liquor. In this method, as an anode, a substrate surface-coated with a oxide of ruthenium, iridium, platinum or palladium, is used. Specifically, a three-dimensional mesh electrode composed of a plurality of expanded-metals is disclosed. Further, WO97/41295 discloses a method for electrolytically producing a polysulfide cooking liquor by the present inventors. In this method, as the anode, a porous anode at least made of carbon is used, particularly an integrated body of carbon fibers having a diameter of from 1 to 300 .mu.m is used.
It is an object of the present invention to obtain polysulfides at a high concentration by an electrolytic method from sulfide ions in a solution, particularly to produce a cooking liquor containing polysulfide sulfur at a high concentration from white liquor in a pulp production process at a high selectivity with a low electrolytic power while minimizing by-production of thiosulfate ions. Further, it is an object of the present invention to provide a method for producing a polysulfide cooking liquor under such a condition for the electrolysis operation that the pressure loss is small.